This invention is concerned with plugs for use in securing a screw in a previously formed hole in a work piece.
Plastic plugs, which are inserted into a hole in a work piece, to receive a conventional screw to secure the screw to the work piece are well known.
It is usual to use different plastic plugs for different work pieces, in particular for securing screws into a blind hole in a hard work piece, such as brick or concrete, into a blind hole in a soft friable work piece, such as blown concrete, or into a hole extending right through a sheet form work piece.
Attempts have been made to provide a plug which is suitable for use in all these three situations, but the requirements for a fastener to work satisfactorily in all three conflict with one another.
For a plug which is to secure a screw in a hole in rigid work piece, the plug is conventionally of uniform outside diameter so that it may readily be inserted in the hole. A central bore, to receive the screw is usually provided with ridges so that on the screw being driven into the plug the screw engages the plug firmly, and a small amount of expansion of the plug takes place to grip the hole firmly. Use of such a plug in a friable material will usually not provide a firm fixture as the expansion of the plug is inadequate to hold the plug securely in the material.
For a plug which is to secure a screw in a hole through a sheet form material, the plug usually comprises a tail portion connected to a head portion of the plug by a series of legs, and the screw is intended to pass freely through the bore of the plug until it reaches the tail portion, and then to draw the tail portion towards the head portion to force the legs to bend outwardly to secure the plug to the work piece. It will be realized that the provision of ridges in the bore of the plug (to enable the plug to be used, as above, in rigid material) will prevent easy passage of the screw from the head portion to the tail portion.
A plug which is intended to be used in the two above situations is described in WO/9008265.
This plug comprises a head portion having an enlarged flange adapted to engage the work piece and a generally cylindrical portion, tapering outwards slightly from the flange, a cylindrical tail portion of substantially the same diameter as the smallest part of the head portion and four legs interconnecting the tail portion and the head portion.
These legs are in two pairs, one pair of opposed legs of approximately a semicircular cross section, and another pair of opposed legs of somewhat smaller cross section.
This plug will operate successfully in a hole in rigid material, and will also operate in a hole in sheet form material of appropriate thickness.
However in soft friable material, the plug will not generally operate satisfactorily: the plug will not expand adequately into the friable material simply on insertion of a screw into the plug, and if the screw is driven further, the rigidity of the larger pair of the legs is such that only a small amount of lateral expansion, in a manner comparable to that taking place when used with sheet material, will take place, and a secure fastening cannot be ensured.
It is one of the objects of the present invention to provide a plug for use in securing a screw in a previously formed hole in a work piece which is useful in a wide variety of circumstances.